Ungalvanized (ferriferrous) and galvanized (zinciferrous) steel sheets and other components, used, for example, in the automotive and construction industries, are usually painted to both protect the steel against rust and for appearance purposes. It is well known that paint does not perform well on uncoated steel or galvanized steel surfaces, i.e. adhesion characteristics are poor, often resulting in blistering of the paint upon aging, and resistance to corrosion is generally unsatisfactory. Accordingly, both steel and galvanized steel surfaces are pretreated prior to painting, usually by the application of a protective coating such as a zinc phosphate coating.
For many years, zinc phosphate coatings were applied by contacting the steel or galvanized steel surfaces, which were previously cleaned and frequently activated, with a solution containing zinc ions and phosphoric acid at elevated temperatures, e.g. in the range of 125.degree. to 190.degree. F. While the resulting zinc phosphate coatings proved very satisfactory for use under paint, the energy requirements to maintain the treating bath at such temperatures became increasingly expensive as the cost of energy climbed.
Therefore, so-called low temperature baths were developed, i.e. baths having temperatures of 110.degree. F. or less. However, the zinc phosphate coatings resulting from such low temperature baths tended to be coarse and powdery, and in general were much less satisfactory than zinc phosphate coatings produced from higher temperature baths.